Ebola viruses cause acute, lethal hemorrhagic fevers for which no human-use vaccines or treatments currently exist. Knowledge about the immune mechanisms mediating protection is limited. The membrane-anchored glycoprotein (GP) is the only viral protein known to be on the surfaces of virions and infected cells, and is presumed to be responsible for receptor binding and fusion of the virus with host cells. As a result, Ebola GP may be an important target of protective antibodies. However, the contribution of antibodies to Ebola GP in disease resistance has been controversial. Negligible serum titers of neutralizing antibodies in convalescent patients, together with inconsistent results in achieving protection through experimental transfers of immune sera to animals (C. J. Peters and J. W. LeDuc, J. Infect. Dis. 179 (Suppl. 1), ix, 1999; V. V. Mikhailov et al., Vopr. Virusol. 39, 82, 1994) have led to suggestions that antibodies to Ebola GP cannot confer protection to Ebola virus (L. Xu et al., Nature Med. 4, 37, 1998).
The role of anti-GP antibodies in protection is further confounded by the observation that Ebola GP occurs in several forms. The transmembrane glycoprotein of Ebola viruses is unusual in that it is encoded in two open reading frames. Expression of GP occurs when the 2 reading frames are connected by transcriptional or translational editing (Sanchez et al., Proc. Natl. Acad. Sci. USA 93, 3602-3607, 1996; Volchkov et al., Virology 214, 421-430, 1995). The unedited GP mRNA produces a non-structural secreted glycoprotein (sGP) that is synthesized in large amounts early during the course of infection (Volchkov et al., 1995, supra; Sanchez et al., 1996, supra; Sanchez et al., J. Infect. Dis. 179 (suppl. 1, S164, 1999). Following editing, the virion-associated transmembrane glycoprotein is proteolytically processed into 2 disulfide-linked products (Sanchez et al., J. Virol. 72, 6442-6447, 1998). The amino-terminal product is referred to as GP1 (140 kDa) and the carboxy-terminal cleavage product is referred to as GP2 (26 kDa). GP1 and membrane-bound GP, covalently associate to form a monomer of the GP spike found on the surfaces of virions (V. E. Volchkov et al., Proc. Natl. Acad. Sci. U.S.A. 95, 5762, 1998; A. Sanchez et al., J. Virol. 72, 6442, 1998). GP1 is also released from infected cells in a soluble form (V. E. Volchkov. et al., Virology 245, 110, 1998). sGP and GP1 are identical in their first 295 N-terminal amino acids, whereas the remaining 69 C-terminal amino acids of sGP and 206 amino acids of GP1 are encoded by different reading frames. It has been suggested that secreted GP1 or sGP may effectively bind antibodies that might otherwise be protective (Sanchez et al., 1996, supra; Volchkov et al. 1998, supra).
Ebola virus GP is a type I transmembrane glycoprotein. Comparisons of the predicted amino acid sequences for the GPs of the different Ebola virus strains show conservation of amino acids in the amino-terminal and carboxy-terminal regions with a highly variable region in the middle of the protein (Feldmann et al., Virus Res. 24: 1-19, 1992). The GP of Ebola viruses are highly glycosylated and contain both N-linked and O-linked carbohydrates that contribute up to 50% of the molecular weight of the protein. Most of the glycosylation sites are found in the central variable region of GP.
Other studies have also demonstrated limited efficacy of passively transferred polyclonal antibodies in protection against Ebola challenge (Mikhailov et al, 1994, Voprosi Virusologii, 39, 82-84; Jahrling et al., 1996, Arch Virol, 11S, 135-140; Jahrling et al., 1999, J Infect Dis, 179 (Suppl 1), S224-234; Kudoyarova-Zubavichene et al., 1999, J Infect Dis, 179 (Suppl 1), S218-223). However, it is difficult to determine the effective therapeutic dose of antibodies in different preparations of polyclonal antibodies. Efforts to identify the role of antibodies in protection led to the isolation of monoclonal antibodies from mice vaccinated with Ebola GP (for instance, co-pending patent application Ser. No. 09/650,086; and Wilson et al. Science 287, 1664, 2000), and from convalescent people (Maruyama et al. J. Infect. Dis. 179 (suppl 1), S235, 1999; Maruyama et al. J. Virol. 73, 6024, 1999; Parren et al. J. Virol 76, 6408, 2002). These were tested in rodents and protected against lethal infection (Wilson et al. Science 287, 1664, 2000; Parren et al. J. Virol 76, 6408, 2002).